USE AND DISUSE, REVERSION, AND HYBRIDISM. 73 



are never exactly alike. It may also be re- 

 marked that in large families the intermediate 

 children sometimes resemble each other more 

 closely than those which are born in succession. 

 It would be highly interesting to ascertain at 

 what period of foetal life the special charac- 

 teristics of the individual are impressed upon 

 it ; but it is nearly impossible to answer such 

 a question satisfactorily in the present state of 

 science, which has not yet succeeded in throw- 

 ing any real light upon the mystery of the 

 origin of life. 



The more important differences which w r e 

 notice in our domesticated animals and culti- 

 vated plants, and in local races in a state of 

 nature, are hereditary, and being thus perpe- 

 tuated, form breeds or races. The instinct of 

 most animals generally leads them to pair with 

 those most like them,* where perfect freedom of 

 choice exists ; and the greater their unlikeness, 

 the greater is their unwillingness to pair. Thus 

 races, once formed, tend to become per- 

 petuated and differentiated. 



* This is not an invariable rule, and the contrary is frequently 

 asserted ; but compare Darwin, "Animals and Plants under Domestica- 

 tion," vol. ii. pp. 102 104. 



